As climate change shifts the timing of life-cycle events, mismatches between species threaten wildlife from whales to birds and butterflies
En route to Arctic breeding grounds, western sandpipers feed in Alaska’s Copper River Delta. If they arrive after peak abundance of their insect prey, the birds and their chicks can suffer. (Photo by Yva Momatiuk and John Eastcott/Minden Pictures)
FROM THE ICONIC SIGHT of birds migrating in formation to the unfurling of buds as spring’s “green wave” starts its northward roll, timing is the pulse that underpins the natural world’s recurring life-cycle events. Its study and processes go by the term phenology—from the ancient Greek phaino, meaning “to come into view,” and logos, meaning “to study.” But to 21st century scientists, phenology also is a fingerprint of accelerating climate change as earlier springs, shrinking snowpack and other consequences of warming impact the timing of budding, blooming, nesting, migrating and a host of other seasonal events across the planet.
In a 2018 global synthesis published in Nature Climate Change, Yale University ecologist Jeremy Cohen and his colleagues reviewed 127 studies and 1,011 phenological time series (long-term records on the timing of seasonal behaviors such as breeding) for 475 animal species. They concluded that the timing of such life-cycle events has, on average, advanced nearly three days per decade just since 1950. Their findings confirmed what scientists—along with gardeners, birdwatchers and other observers of the natural world—already had been documenting for many years.
Like moving to cooler habitats, shifting the timing of life-cycle events may help some species adapt to a warming planet. “There’s been a lot of work looking at species’ ranges shifting in space, northward or poleward or to higher elevations,” says National Wildlife Federation Chief Scientist Bruce Stein. While harder to document, he says, phenological shifts “may enable species to continue persisting or thriving in the same places they currently live.”
Support for that argument comes from an ambitious effort by researchers at the University of California–Berkeley who have been resurveying locations that famed scientist Joseph Grinnell visited a century ago to make field observations of California’s vertebrate wildlife. Quantifying differences in signs of breeding activity between the original surveys and recent resurveys, the researchers found that, on a community-wide basis, the birds are now nesting five to 12 days earlier. During the same period, temperatures have warmed by approximately 1 degree C. By breeding about a week earlier, the birds are nesting at temperatures about 1 degree cooler—similar to temperatures that existed when they bred in Grinnell’s day a century ago.
But as temperatures rise, not all species are dancing to a single accelerating beat. In their review, Cohen and his colleagues found, for example, that smaller-bodied, cold-blooded and lower-trophic-level animals such as insects are advancing their phenology much faster than larger, warm-blooded, higher-trophic-level animals such as birds. “This may be because they have simpler biological processes to adjust quickly in reaction to temperature changes,” he says.
No plant or animal species dances alone, however. Whether herbivores and plants, migratory birds and insects or hosts and parasites, the natural world is built on species interactions, including interdependencies in which ecological partners may fall out of step with each other.
“This is especially a problem where you have very specialized interactions,” Stein says. He cites the endangered Karner blue butterfly, whose larvae feed exclusively on wild blue lupine. The species already had declined significantly when, in 2012, the extreme warmth of an early spring at Indiana Dunes National Park—the butterfly’s southernmost outpost—induced its larvae to emerge exceptionally early before lupine was ready for the caterpillars to eat. Drought conditions later in summer stressed the lupine itself, stressing the butterflies further. Though other factors may also have played a role, a phenological mismatch between the Karner blue and its host plant has contributed to the butterfly’s local extinction from Indiana Dunes.
Animals that migrate long distances face special risks. Nowhere is this more evident than in the Arctic, which is warming about four times more rapidly than the planet as a whole. Every year, millions of shorebirds such as phalaropes, plovers and sandpipers travel thousands of miles from South America, Asia, Australia and other wintering grounds to breed in the Arctic. Since the 1970s, overall populations of these Arctic-breeding shorebirds have declined dramatically—and recent research suggests that a phenological mismatch between the birds and their insect prey on the breeding grounds may exacerbate these declines.
Because they are hardwired to begin their migrations by the level of daylight where they winter, the birds’ departure from wintering grounds is relatively fixed. Yet the timing of snowmelt at their destinations—which cues the abundant but brief pulse of protein-rich insects the birds need to eat—is now highly variable. “In years when spring arrives exceptionally early, shorebirds arrive late to the party, after the food has already been served,” says Alaska-based U.S. Fish and Wildlife biologist Sarah Saalfeld, who researches Arctic-breeding shorebirds. “This abundant food source is especially important to chicks that must feed themselves soon after hatching.”
A 14-year study by Saalfeld and her colleague Rick Lanctot near Utqiaġvik (formerly Barrow), Alaska, investigated the timing of egg laying for eight Arctic-breeding shorebird species and how it related to the snowmelt that triggers insect emergence. Over the course of the study, snowmelt around their site advanced by 11 days—a drastic change. In turn, the shorebirds advanced their nesting by 0.1 to 0.9 days a year depending on the species, but no species’ nesting date kept pace with advancing snowmelt, suggesting that chicks were getting less food. Over a period of seven years, the researchers found that only 20 to 54 percent of dunlin and pectoral sandpiper broods had enough food for average growth, and fewer than half the chicks of the other six species had enough food. In addition, dunlin chick survival was substantially lower in clutches that hatched during times when insects were less abundant.
In marine ecosystems, the exact times animals arrive at and depart from seasonal habitats are inherently more difficult to observe. “If you only studied the terrestrial phenology literature, you’d think that the timing of all things spring is happening earlier,” says ecologist Michelle Staudinger of the U.S. Geological Survey and the Northeast Climate Adaptation Science Center at the University of Massachusetts–Amherst. But off New England, the Gulf of Maine—one of the world’s fastest-warming ecosystems—is yielding surprising findings. “Some species are shifting earlier, some later and some not at all, at least that we can detect,” she says.
A particularly urgent challenge for Gulf of Maine researchers is the endangered North Atlantic right whale. Only about 340 individuals remain, and scientists fear that vessel strikes, fishing gear entanglements and other threats may drive the species to extinction within three decades.
In recent years, the migratory whales have reduced their use of most traditional feeding grounds in the Gulf of Maine, likely because of climate-related reductions in their zooplankton prey. The exception is Cape Cod Bay, where biologist Laura Ganley and others from the New England Aquarium and Center for Coastal Studies documented an increase in winter-through-spring right whale abundance between 1998 and 2017. Moreover, the whales are also staying longer, even as spring’s onset trends earlier.
Their prey, however, does not seem to be responding similarly. “Scientists expected years with earlier springs to have higher densities of right whale prey,” Ganley says. Instead, in 2022, she and her colleagues reported that years with earlier springs had lower zooplankton abundance, but still higher right whale abundance—worrisome signs of a phenological mismatch. “Declines in the health of right whales observed in recent years could be a warning sign that the whales’ nutritional needs are not being met,” Ganley says.
Detecting phenological changes can be hugely challenging, especially in places like the Arctic and the oceans. But at the same time, volunteers across the country are helping scientists uncover such changes by regularly recording life-cycle events for the plants and animals they see every day, from backyard trees to frogs and songbirds. These volunteer observers are heirs to the tradition of 19th century naturalist Henry David Thoreau, whose meticulous records for hundreds of plant and animal species around Massachusetts’ Walden Pond remain invaluable to researchers, who have used them to document today’s earlier arrival of spring. In 2022 alone, volunteer participants in Nature’s Notebook—a program of the USA National Phenology Network—submitted nearly 4 million reports on the timing of seasonal phenological events for 1,650 species.
Two decades ago, University of Montana wildlife biologist Scott Mills didn’t have to look far to notice a species—the snowshoe hare—that was not adapting to changes brought on by warming. “I was seeing more and more of these white light bulbs hopping around on a brown, snowless background,” he recalls. Along with 20 other vertebrate species worldwide, most of the region’s snowshoe hares molt twice a year, exchanging brown and white coats in order to fade into seasonal backgrounds and elude predators. But by the turn of the 21st century, as snow duration waned, the timing of the hares’ molts was failing them.
Mills and his colleagues found that for every week the hares’ coats aren’t matched to the background, their mortality increases by 7 to 12 percent. That now happens for a week or two a year, but by century’s end, it could increase to nearly 10 weeks, leading the hare toward local extinction—and the threatened Canada lynx, which eats almost nothing else, toward starvation and reproductive failure. Other predators, along with herbivores dependent on timing and palatability of vegetation the hare eats, would also be at risk. “Hares underlie a lot of the boreal ecosystem,” Mills says.
But Mills also sees glimmers of hope. Thanks to a mutant Agouti gene—a major driver of coat color—some snowshoe hares in more-temperate, less-snowy areas such as coastal Oregon began having brown winter coats after the last ice age ended. “Now you see winter browns and winter whites living together,” he says. Although this occurs in less than 5 percent of the hare’s range today, Mills believes that such polymorphic zones are likely to spread as snow cover continues to wane.
Whether snowshoe hares can adapt fast enough is a big question. But it’s not the only one. Mills says the recipe for the hares’ evolutionary rescue also requires a “special sauce” —large, diverse populations in big, interconnected habitats, with few other stressors. It’s a tall order.
“Evolution isn’t a magic bullet,” Mills says. He and other scientists agree that the vitality—and even survival—of a host of wildlife species depends on a drastic reduction in humans’ carbon footprint. But phenological adaptations may buy time, at least for some species, as climate change scrambles the rhythms of the natural world.
The National Wildlife Federation is at the forefront of the new field of climate adaptation, working with federal and state agencies and other organizations to make conservation projects more resilient by incorporating climate change into plans to protect species and ecosystems. Learn more at nwf.org/climatesmart.
Leslie Allen is a science writer based in Washington, D.C.
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